Generally, conventional release coating compositions are solvent-based polymers. They are typically prepared using solution polymerization in organic solvents. In an attempt to meet many environmentally driven standards to reduce or replace volatile organic solvents, efforts have been directed towards the development of waterborne release coatings and low adhesion backsize coatings. Furthermore, reducing and/or eliminating solvents has added economical advantage. However, the aqueous release coatings described in the literature are not completely solvent-free systems. Many of the existing waterborne release coatings involve processes where solvents are used during the polymerization. These processes then require an additional step of distillation or other process(es) to remove the solvents.
For examples, release agents can include vinyl acetate-stearyl maleate copolymers and octadecyl acrylate-acrylic acid copolymers. Such release agents are prepared by mixing the copolymers, which are prepared and contained within organic solvents, with isopropanol, water, and morpholine followed by distillation in order to remove the organic solvents.
In another example, a low adhesion backsize is described comprising a polymer of octadecyl acrylate, (meth)acrylic acid, (meth)acrylonitrile, and methyl (meth)acrylate and is obtained by the polymerization of the monomers in one or more organic solvents. This polymer solution can then be converted into an aqueous dispersion. The method involves the addition of water and morpholine to an organic solvent solution of the polymer followed by the removal of the organic solvent by distillation.
In yet another process, a release polymer is obtained by polymerizing a higher alkyl (meth)acrylate ester monomer (stearyl methacrylate is preferred), a carboxyl supplying monomer, and an (meth)acrylate derived hardening monomer in an organic solvent. The aqueous backsize composition is prepared by adding a base to a solution of the terpolymer in an organic solvent, followed by the addition of a thickener and an aqueous solvent mixture.
As indicated, the use of solvents is not desirable due to environmental, economic, and safety reasons.
Attempts have been made at preparing aqueous-based dispersions of long alkyl chain branched polymers by the emulsion polymerization of monomer systems comprising long alkyl chain vinyl monomers such as octadecyl acrylate, stearyl methacrylate, vinyl stearate, etc. However, because of the relatively low water solubility of these long chain monomers as compared to short alkyl chain vinyl monomers in aqueous systems, difficulties have been encountered in attempting to incorporate these long alkyl chain vinyl monomers into a polymer structure. This problem is significantly augmented when the short alkyl chain vinyl monomers are soluble in water. Thus, the polymer dispersions or emulsions prepared via such methods do not provide desirable release levels and stable release upon aging.
To overcome the difficulties encountered in attempting to incorporate long alkyl chain vinyl monomers, alternative processes have been attempted wherein a waterborne low adhesion backsize latex comprising polymerized free radically polymerizable monomers of the formula C.sub.n H.sub.2n+1 --P--C.sub.m H.sub.2m --X--CR'=CH.sub.2, wherein n is an integer of about 12 to about 20, P is a divalent linking group, X is either --OCO-- or --NHCO--, m is an integer of about 2 to about 10 and R' is --H or --CH.sub.3. The latices are prepared according to a batch polymerization. Although the latices are waterborne, release values increase significantly after aging, thus indicating less than desirable aging characteristics.
Thus, there is a need to provide water-borne release coatings that are essentially solvent free, while at the same time providing desirable release and aging characteristics.